Switching direct current (DC) to DC voltage converters are used in a variety of applications for converting power at an input voltage into power at a desired output voltage. Such voltage converters are used to power battery chargers, computers, televisions, and many other electronic devices. Switching voltage converters may be broadly classified into isolated and non-isolated topologies, and into regulated and non-regulated categories.
Isolated voltage converters use magnetic coupling, via, typically, a transformer, to transfer power from a primary to a secondary side. One or more switches (e.g., transistors) are used to generate voltage pulses from an input DC voltage source. The resultant alternating current (AC) voltage is supplied to the primary side of the transformer, thereby inducing an AC voltage on the secondary side of the transformer, which may be rectified to provide DC power to a load. The transformer provides galvanic (electrical) isolation between the primary and secondary sides, which has safety advantages particularly for high-power applications.
Non-isolated switching voltage converters may also use magnetics, as provided by inductors, to convert an input voltage into an output voltage, but do not provide galvanic isolation between the input and output. Example non-isolated voltage converters include buck, boost, and buck-boost converters.
Most switching voltage converters are regulated so as to provide a near-constant voltage or current at their outputs. This is accomplished by adapting the conduction timing of the switches that control the power flow through the voltage converter based, at least in part, upon a measured voltage or current at the output of the voltage converter. For example, a measured output voltage falling below a desired reference output voltage might force a change in the switching frequency and or duty cycle of signals that control the switches, such that additional power is transferred from the input to the output of the voltage converter. The switch control signals are typically generated by a controller that implements a closed-loop control technique, so as to regulate the output voltage at a near constant level.
Some electric loads do not require tight regulation, and the voltage converters used to supply such loads may forgo complex closed-loop control techniques and the associated measurement sensors required by closed-loop control. Switched capacitor converters (SCCs) represent one class of voltage converters that may be operated in an unregulated mode. SCCs are commonly used to step down an input voltage by a fixed ratio, e.g., 2:1, 4:1. Such step-down SCCs effectively function as voltage dividers. In an alternate configuration, an SCC may operate in a step-up mode, wherein an input voltage is multiplied by a fixed ratio, e.g., providing a step-up of 2, 4. SCCs provide low-impedance, high-efficiency voltage conversion for applications where regulation is not required.
SCC operation depends upon using switches to transfer energy among several capacitors. Within an SCC circuit, there is a trade-off between switch frequency and capacitor size. To use reasonably small capacitors, the switch frequencies must be relatively high. High switching frequencies typically require switches that have relatively low current flow capabilities. Hence, usage of SCCs is often limited to applications wherein the SCC load requires relatively low or moderate current (power).
A switching voltage converter that exhibits the relative simplicity and efficiency of an SCC, but that is able to transfer high current levels, is desired.